Tire is a complex body of fiber/steel/rubber and generally has a structure as illustrated in FIG. 1. The body ply, which is denoted by the numeral 6 of FIG. 1 and also called as “carcass”, is a cord layer included in a tire as a core reinforcement material that supports the load of a whole vehicle, maintain the shape of the tire, stands against the shock, and needs to have excellent fatigue resistance against bending and stretching movements during the drive. The tire cord for such a body ply is usually made of a synthetic fiber material, including polyester such as poly(ethyleneterephthalate).
The cords made of synthetic fibers, thanks to their high tenacity, greatly contribute to the enhanced durability of tires, yet with a problem that they may have deterioration in elasticity and dimensional stability after vulcanization of tires because of their high shrinkage rate under heat. To offset this, there have been made many studies to improve the dimensional stability of cords by adopting an additional process such as post cure inflatation (PCI). Particularly, high tenacity fibers for industrial use exhibit high tenacity as achieved by increasing the draw ratio at low speed but still remain requisitely in need of the PCI process due to their high heat shrinkage rate and low elasticity.
As the ultrahigh speed spinning technology has recently been grafted onto the preparation method of tire cords, it is now possible to prepare polyester tire cords having high-modulus low-shrinkage (HMLS) properties without a need for the PCI process.
In this regard, undrawn fibers with high crystallinity are necessarily used in order to prepare HMLS tire cords. However, the undrawn fibers with high crystallinity have a relatively narrow drawing range and thus tend to break by uneven drawing or friction when they are drawn under conditions of high speed and high draw ratio in an ultrahigh speed spinning system. On this account, applying the undrawn fibers with high crystallinity to the ultrahigh speed spinning system is bound to limitations in the draw ratio and ends up with insufficient drawing, causing great deterioration in the tenacity of the drawn fibers. Particularly, the preparation of drawn fibers with high fineness of 2,000 denier or greater and tire cords entails greater deterioration of properties due to its limitations in the spinning process and the cooling process and thus results in a failure to produce tire cords with uniform properties.
This problem makes difficult to meet the demand for providing tire cords with high fineness and uniform, excellent properties that is attendant to the recent increase in the use of radial tires. Furthermore, the tire cords with high fineness are usually applied to large-sized tires and thus susceptible to deterioration of tenacity under a long-term vulcanization process as a result of using an excess of rubber per vulcanization time. This further leads to a great deterioration in the properties such as tenacity, still there is no suggestion to improve the problem.